Leadless Chip Carriers (LCCs) are well known in fabricating techniques involving surface mounted devices. It is desirable in such assemblies that the chip carrier be separated from the circuit board in order to allow removal of any solder flux or spattered solder which has found its way underneath the carrier during the fabricating process. It is also desirable to have elongated solder joints between the conductive pads on the carrier and on the board to avoid solder joint failures caused by different thermal coefficients of expansion between the chip carrier and the circuit board. By increasing the solder joint height, thereby separating the chip carrier and the substrate, fatigue failures due to these different coefficients of temperature expansion can be reduced because an elongated solder connection is more compliant and can bend somewhat when stressed and will not fracture as readily as a shorter solder joint.
Typically, in mounting leadless chip carriers, solder paste is first deposited on the circuit board connector pads. The thickness of the solder paste is generally equal to the desired spacing between the chip carrier and the board. The connector pads on the board and the solder paste are individually aligned with the conductive pads on the chip carrier. The solder is heated causing the solder paste to reflow and form solder joints between the connector pads on the chip carrier and the corresponding connector pads on the circuit board. It has been found, however, that problems exist in forming solder posts which are sufficiently high to provide the clearance desired for cleaning under the LCCs and which are sufficiently compliant to avoid fatigue and fracture under various temperature conditions. These problems are basically related to the fact that the mass of the leadless chip carrier is large relative to the mass of the solder paste so that it has a tendency to compress or squash the molten solder. The problem is exacerbated by the present movement toward utilization of high pad count packages in small package configurations. Today, chip carriers with pad counts of 132 to 256 pads are commonly used. This requirement for small, high pad count packages imposes further limitations on the spacing and the size of the pads with center-to-center spacings of 20 mills or less being quite common. Obviously, with such narrow spacings, the cross section of the pad and, hence, the cross section of the solder paste is concurrently reduced. This increase the compression or squashing of the solder during reflow and reduces the spacing between the carrier and the board as well as producing spatter and possible shorting by the spattered solder between adjacent pads.
Applicant has found that elongated solder post attachment of leadless chip carriers may be readily achieved, and the problem of solder compression or squashing during solder reflow due to the mass of the chip carrier may be virtually eliminated by providing a temporary support for the solder paste prior to and during reflow of the solder. This temporary support surrounds the solder paste during reflow and supports the chip carrier thus allowing soldering to the chip carrier pads without compression of the solder. After cooling and hardening of the solder, the support element is removed by chemical processes presently to be described.